In a typical stator of a generator with preformed coil windings, a wedge is used to keep the coils fixed inside their respective stator slots. In wind power applications, preformed coils are preferred to other alternatives due to their high reliability. Because of their solid preformed structure, these coils need to be inserted radially into the slots.
Therefore, the slots need to be completely open near the air gap. The wedge can be made of magnetic or non-magnetic material, while the non-magnetic wedge is preferred due to a higher reliability.
To insert the wedge above the coils and close the open slots, the stator laminations have recesses inside the stator slots near the slot opening. It can be theoretically shown that these openings, in case of a non-magnetic wedge, increase the cogging and ripple torque due to magnetic saturation of the narrow corners of the stator teeth. Furthermore, in case the non-magnetic wedge is used, the wedge material has a low thermal conductivity and thereby insulates the winding from the airflow in the air gap and decreases the cooling efficiency of the generator. Finally, in case of non-magnetic wedge, the ripple and cogging torque can be significantly large which requires some ripple torque minimization techniques like shaping the permanent magnets in a permanent magnet generator.